Cyclin C/CDK8 and cyclin H/CDK7/p36 are biochemically distinct CTD kinases.
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Phosphorylation of the carboxyl-terminal domain (CTD) of RNA polymerase II is important for basal transcriptional processes in vivo and for cell viability. Several kinases, including certain cyclin-dependent kinases, can phosphorylate this substrate in vitro. It has been proposed that differential CTD phosphorylation by different kinases may regulate distinct transcriptional processes. We have found that two of these kinases, cyclin C/CDK8 and cyclin H/CDK7/p36, can specifically phosphorylate distinct residues in recombinant CTD substrates. This difference in specificity may be largely due to their varying ability to phosphorylate lysine-substituted heptapeptide repeats within the CTD, since they phosphorylate the same residue in CTD consensus heptapeptide repeats. Furthermore, this substrate specificity is reflected in vivo where cyclin C/ CDK8 and cyclin H/CDK7/p36 can differentially phosphorylate an endogenous RNA polymerase II substrate. Several small-molecule kinase inhibitors have different specificities for these related kinases, indicating that these enzymes have diverse active-site conformations. These results suggest that cyclin C/CDK8 and cyclin H/CDK7/p36 are physically distinct enzymes that may have unique roles in transcriptional regulation mediated by their phosphorylation of specific sites on RNA polymerase II. (+info)
GAL4 is regulated by the RNA polymerase II holoenzyme-associated cyclin-dependent protein kinase SRB10/CDK8.
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Phosphorylation of the yeast transcription factor GAL4 at S699 is required for efficient galactose-inducible transcription. We demonstrate that this site is a substrate for the RNA polymerase holoenzyme-associated CDK SRB10. S699 phosphorylation requires SRB10 in vivo, and this site is phosphorylated by purified SRB10/ SRB11 CDK/cyclin in vitro. RNA Pol II holoenzymes purified from WT yeast phosphorylate GAL4 at sites observed in vivo whereas holoenzymes from srb10 yeast are incapable of phosphorylating GAL4 at S699. Mutations at GAL4 S699 and srb10 are epistatic for GAL induction, demonstrating that SRB10 regulates GAL4 activity through this phosphorylation in vivo. These results demonstrate a function for the SRB10/ CDK8 holoenzyme-associated CDK that involves regulation of transactivators by phosphorylation during transcriptional activation. (+info)
Transcription: Common cofactors and cooperative recruitment.
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Mammalian counterparts of the yeast SRB/MED transcriptional 'mediator' complex have recently been identified. These complexes define a common cofactor requirement for diverse transcriptional activators and underscore the conserved nature of the transcriptional machinery among eukaryotic organisms. (+info)
Multiple signals regulate GAL transcription in yeast.
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Gal4p activates transcription of the Saccharomyces GAL genes in response to galactose and is phosphorylated during interaction with the RNA polymerase II (Pol II) holoenzyme. One phosphorylation at S699 is necessary for full GAL induction and is mediated by Srb10p/CDK8 of the RNA Pol II holoenzyme mediator subcomplex. Gal4p S699 phosphorylation is necessary for sensitive response to inducer, and its requirement for GAL induction can be abrogated by high concentrations of galactose in strains expressing wild-type GAL2 and GAL3. Gal4p S699 phosphorylation occurs independently of Gal3p and is responsible for the long-term adaptation response observed in gal3 yeast. SRB10 and GAL3 are shown to represent parallel mechanisms for GAL gene induction. These results demonstrate that Gal4p activity is controlled by two independent signals: one that acts through Gal3p-galactose and a second that is mediated by the holoenzyme-associated cyclin-dependent kinase Srb10p. Since Srb10p is regulated independently of galactose, our results suggest a function for CDK8 in coordinating responses to specific inducers with the environment through the phosphorylation of gene-specific activators. (+info)
A regulatory shortcut between the Snf1 protein kinase and RNA polymerase II holoenzyme.
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RNA polymerase II holoenzymes respond to activators and repressors that are regulated by signaling pathways. Here we present evidence for a "shortcut" mechanism in which the Snf1 protein kinase of the glucose signaling pathway directly regulates transcription by the yeast holoenzyme. In response to glucose limitation, the Snf1 kinase stimulates transcription by holoenzyme that has been artificially recruited to a reporter by a LexA fusion to a holoenzyme component. We show that Snf1 interacts physically with the Srb/mediator proteins of the holoenzyme in both two-hybrid and coimmunoprecipitation assays. We also show that a catalytically hyperactive Snf1, when bound to a promoter as a LexA fusion protein, activates transcription in a glucose-regulated manner; moreover, this activation depends on the integrity of the Srb/mediator complex. These results suggest that direct regulatory interactions between signal transduction pathways and RNA polymerase II holoenzyme provide a mechanism for transcriptional control in response to important signals. (+info)
Genetic analysis of the role of Pol II holoenzyme components in repression by the Cyc8-Tup1 corepressor in yeast.
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The Cyc8-Tup1 corepressor complex is targeted to promoters by pathway-specific DNA-binding repressors, thereby inhibiting the transcription of specific classes of genes. Genetic screens have identified mutations in a variety of Pol II holoenzyme components (Srb8, Srb9, Srb10, Srb11, Sin4, Rgr1, Rox3, and Hrs1) and in the N-terminal tails of histones H3 and H4 that weaken repression by Cyc8-Tup1. Here, we analyze the effect of individual and multiple mutations in many of these components on transcriptional repression of natural promoters that are regulated by Cyc8-Tup1. In all cases tested, individual mutations have a very modest effect on SUC2 RNA levels and no detectable effect on levels of ANB1, MFA2, and RNR2. Furthermore, multiple mutations within the Srb components, between Srbs and Sin4, and between Srbs and histone tails affect Cyc8-Tup1 repression to the same modest extent as the individual mutations. These results argue that the weak effects of the various mutations on repression by Cyc8-Tup1 are not due to redundancy among components of the Pol II machinery, and they argue against a simple redundancy between the holoenzyme and chromatin pathways. In addition, phenotypic analysis indicates that, although Srbs8-11 are indistinguishable with respect to Cyc8-Tup1 repression, the individual Srbs are functionally distinct in other respects. Genetic interactions among srb mutations imply that a balance between the activities of Srb8 + Srb10 and Srb11 is important for normal cell growth. (+info)
Roles of transcription factor Mot3 and chromatin in repression of the hypoxic gene ANB1 in yeast.
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The hypoxic genes of Saccharomyces cerevisiae are repressed by a complex consisting of the aerobically expressed, sequence-specific DNA-binding protein Rox1 and the Tup1-Ssn6 general repressors. The regulatory region of one well-studied hypoxic gene, ANB1, is comprised of two operators, OpA and OpB, each of which has two strong Rox1 binding sites, yet OpA represses transcription almost 10 times more effectively than OpB. We show here that this difference is due to the presence of a Mot3 binding site in OpA. Mutations in this site reduced OpA repression to OpB levels, and the addition of a Mot3 binding site to OpB enhanced repression. Deletion of the mot3 gene also resulted in reduced repression of ANB1. Repression of two other hypoxic genes in which Mot3 sites were associated with Rox1 sites was reduced in the deletion strain, but other hypoxic genes were unaffected. In addition, the mot3Delta mutation caused a partial derepression of the Mig1-Tup1-Ssn6-repressed SUC2 gene, but not the alpha2-Mcm1-Tup1-Ssn6-repressed STE2 gene. The Mot3 protein was demonstrated to bind to the ANB1 OpA in vitro. Competition experiments indicated that there was no interaction between Rox1 and Mot3, indicating that Mot3 functions either in Tup1-Ssn6 recruitment or directly in repression. A great deal of evidence has accumulated suggesting that the Tup1-Ssn6 complex represses transcription through both nucleosome positioning and a direct interaction with the basal transcriptional machinery. We demonstrate here that under repressed conditions a nucleosome is positioned over the TATA box in the wild-type ANB1 promoter. This nucleosome was absent in cells carrying a rox1, tup1, or mot3 deletion, all of which cause some degree of derepression. Interestingly, however, this positioned nucleosome was also lost in a cell carrying a deletion of the N-terminal coding region of histone H4, yet ANB1 expression remained fully repressed. A similar deletion in the gene for histone H3, which had no effect on repression, had only a minor effect on the positioned nucleosome. These results indicate that the nucleosome phasing on the ANB1 promoter caused by the Rox1-Mot3-Tup1-Ssn6 complex is either completely redundant with a chromatin-independent repression mechanism or, less likely, plays no role in repression at all. (+info)
Characterization of CAF4 and CAF16 reveals a functional connection between the CCR4-NOT complex and a subset of SRB proteins of the RNA polymerase II holoenzyme.
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The CCR4-NOT transcriptional regulatory complex affects transcription both positively and negatively and consists of the following two complexes: a core 1 x 10(6) dalton (1 MDa) complex consisting of CCR4, CAF1, and the five NOT proteins and a larger, less defined 1.9-MDa complex. We report here the identification of two new factors that associate with the CCR4-NOT proteins as follows: CAF4, a WD40-containing protein, and CAF16, a putative ABC ATPase. Whereas neither CAF4 nor CAF16 was part of the core CCR4-NOT complex, both CAF16 and CAF4 appeared to be present in the 1.9-MDa complex. CAF4 also displayed physical interactions with multiple CCR4-NOT components and with DBF2, a likely component of the 1.9-MDa complex. In addition, both CAF4 and CAF16 were found to interact in a CCR4-dependent manner with SRB9, a component of the SRB complex that is part of the yeast RNA polymerase II holoenzyme. The three related SRB proteins, SRB9, SRB10, and SRB11, were found to interact with and to coimmunoprecipitate DBF2, CAF4, CCR4, NOT2, and NOT1. Defects in SRB9 and SRB10 also affected processes at the ADH2 locus known to be controlled by components of the CCR4-NOT complex; an srb9 mutation was shown to reduce ADH2 derepression and either an srb9 or srb10 allele suppressed spt10-enhanced expression of ADH2. In addition, srb9 and srb10 alleles increased ADR1(c)-dependent ADH2 expression; not4 and not5 deletions are the only other known defects that elicit this phenotype. These results suggest a close physical and functional association between components of the CCR4-NOT complexes and the SRB9, -10, and -11 components of the holoenzyme. (+info)